1. Field of the Invention
The present invention relates to an apparatus and a method of measuring concentration of fuel, and more particularly to, an apparatus and a method capable of measuring concentration of fuel easily and precisely.
2. Description of Related Art
With the rapid development of industry, the consumption of conventional energy source such as coal, petroleum, and natural gas is increasingly high, and due to the limited storage of natural energy source, novel alternative energy source may be researched and developed to substitute the conventional energy source, and the fuel cell is taken as an important and practical choice.
In brief, the fuel cell is substantially a power generator that converts chemical energy into electric energy by utilizing the reverse reaction of the water electrolysis. The proton exchanging membrane fuel cell mainly includes a membrane electrode assembly (MEA) and two electrode plates. The MEA includes a proton conducting membrane, an anode catalyst layer, a cathode catalyst layer, an anode gas diffusion layer (GDL) and a cathode GDL. The anode catalyst layer and the cathode catalyst layer are respectively disposed on two sides of the proton conducting membrane, and the anode GDL and the cathode GDL are respectively disposed on the anode catalyst layer and the cathode catalyst layer. Furthermore, two electrode plates include an anode and a cathode, which are respectively disposed on the anode GDL and the cathode GDL.
Currently, the common proton exchanging membrane fuel cell is Direct Methanol Fuel Cell (DMFC), which directly takes the methanol aqueous solution as the source for supplying fuel, and generates currents through the relevant electrode reaction between methanol and oxygen. The reaction formulas of the DMFC are shown as follows:Anode: CH3OH+H2O→CO2+6H++6e−Cathode: 3/2O2+6H++6e−→3H2O
In the conventional DMFC, the output stability of the DMFC is significantly influenced by the concentration of the methanol aqueous solution conducted to the anode. When the concentration of the methanol aqueous solution conducted to the anode is not properly controlled, not only efficiency and output power of the DMFC is unstable, but also the membrane electrode assembly (MEA) may be damaged. Accordingly, how to control the concentration of the methanol aqueous solution conducted to the anode within an optimized range is a key issue being researched and developed in this field.
The concentration of the fuel may be controlled by the following steps. First, the concentration of the fuel is measured through sensors directly, and amount of fuel and water are supplied to the DMFC is determined in accordance with the measured concentration. Such manner has already discussed in U.S. Pat. No. 6,589,671 B1, U.S. Pat. No. 6,488,837, US 2002/076589 A1, US 2003/0196913 A1, and WO 01/35478. It is noted that the membrane electrode assembly (MEA) disclosed in U.S. Pat. No. 6,488,837 and US 2003/0196913 A1 is used as sensors to measure the concentration of the methanol aqueous solution directly. In the above-mentioned method, precision of measurement is influenced by the impurities in fuel. Additionally, precision of measurement may also be influenced by aging or instability of the membrane electrode assembly (MEA).
In some prior arts (e.g. U.S. Pat. No. 6,698,278 B2), the concentration of fuel is estimated by measuring temperature and current of the DMFC. In the above-mentioned method, the concentration of fuel is measured without sensors. However, when the method described in U.S. Pat. No. 6,698,278 B2 is applied to different fuel cell systems, a proper calibration is necessary to estimate the concentration of fuel. Other measurements of fuel concentration without using sensors are illustrated in U.S. Pat. No. 6,589,679 and TW 94119975.
Since the concentration of methanol aqueous solution is relevant to the physical properties (e.g. velocity of sound transmitted in methanol aqueous solution, dielectric constant or density of fuel, and so on), some prior arts estimate the concentration of methanol aqueous solution by measuring velocity of sound transmitted in methanol aqueous solution, or estimate the concentration of fuel by measuring dielectric constant or density of fuel (TWI 251954). However, sensors used in the above-mentioned estimation of concentration are very expensive and the precision of the sensors is significantly influenced by bubbles in fuel. Therefore, it is difficult to measure physical properties of fuel precisely because the fuel sampled by the sensors must be static and has no bubbles therein when measuring.
In the conventional measurements of fuel, the concentration of fuel is hard to measure. Additionally, cost of measurements is high and precision of measurements is unstable. Accordingly, a simple and precise method of measuring concentration of fuel is needed.